Before our discussion about the function of Endoplasmic Reticulum we should have some idea about the endoplasmic reticulum. Here these are for you and it is very easy to understand the matter:
What is Endoplasmic Reticulum?
The membrane bound irregular, network-like vacuolar system present in the cytoplasm of eukaryotic cells is called endoplasmic reticulum.
Number and distribution
It is important to know the number and distribution of Endoplasmic Reticulum (ER) before we go for our study about the function of endoplasmic reticulum. Endoplasmic reticulum is found in all eukaryotic cells except ovum, mammalian matured RBC, fungi and some lower plant cells. It extends from the cell membrane to the nuclear membrane but it remains more concentrated in the endoplasm than in the ectoplasm, and hence it is so named. The size and number of ER in a cell depends on the size of the cell.
Origin of ER
The exact origin of ER is uncertain but because of its intimate relationship with cell membrane and nuclear membrane, ER is believed to originate from these membranes.
Structure of ER
ER is made up of a network of different types of membrane bound vacuoles or canaliculi that remain connected with the plasmalemma at one end and nuclear membrane at the other end. The knowledge about the structure of Endoplasmic Reticulum will make it very easy to understand about the function of endoplasmic reticulum. Three types of structures are observed in the ER, those are as follows:
Also named as Cisternae which are wide and flat membrane bound cavities that are parallely arranged like helves. The adjacent cisternae remain communicated. Numerous ribosomes are attached to their surface membrane. So, the cisternae are rough surfaced structures.
These are elongated and branched tube-like structures devoid of ribosomes on their surface ; hence, they have a smooth surface.
These are spherical or oval sac-like cavities.
The tubules remain connected to both cisternae and vesicles, thereby giving a reticular appearance of the organelle. Structurally, the ER may be of two types-rough and smooth. Those containing ribosomes on their surface are called granular or rough endoplasmic reticulum (RER) or ergastoplasm, whereas those devoid of ribosome are called agranular or smooth endoplasmic reticulum (SER).
To know the function of Endoplasmic Reticulum we should kept it in mind that, Obviously the RER is made up of cisternae (that contain ribosomes) mainly, whereas the SER is mainly composed of tubules (devoid of ribosomes). The attachment of ribosomes to the ER-membrane is mediated by special types of glycoproteins called ribophorins. Ribophorins are of two types-ribophorin I and ribophorin II that are present in the RER membrane. They have specific affinity to ribosomes and bind them with the RER. Ribophorins are not found in SER.
The chemical composition is also an important matter before discussing about the function of endoplasmic reticulum. As the ER is a membranous organelle, its chief constituents are lipid and protein. The lipids compose 30-50%, of which the major portion is lecithin and cephalin type of phospholipids. The ribosomes present in RER are made up of RNA and protein. And Now:
The Function of Endoplasmic Reticulum
The function of ER are as follows:
1. Mechanical support
The ER forms a network like structure in the cytoplasm that gives mechanical support to the cytoplasmic matrix.
2. Formation of other organelles
ER helps in the origin of nuclear membrane, Golgi body and microbodies.
3. Exchange of materials
The tubular network of ER is continuous externally with the cell membrane, and extracellular fluid (ECF) flows within it. Thus, the ER provides an 'intracellular circulatory system' that ensures a far greater surface of exposure of the cytoplasm to the extracellular environment. This facilitates exchange of materials between cytoplasm and ECF.
4. Protein synthesis
Protein synthesis is a great function of endoplasmic reticulum. Due to the presence of ribosomes in them, the RER functions as the site of protein synthesis.
5. Lipid synthesis
ER takes part in synthesis of various lipids e.g., fats (triglycerides), steroids, lipoproteins and phospholipids. This function is attributed to SER.
6. Carbohydrate metabolism
The SER is also believed to be involved in glycogenolysis.
The SER is probably concerned with detoxication of drugs and toxins.
8. Transport of information
It is another very important function of endoplasmic reticulum. The SER is especially important for transmission of impulse from the cell membrane into the interior of the cell. Particularly in muscle fibers, the SER is modified into sarcoplasmic reticulum which transmits the impulse received by the cell membrane to the contractile machinery for excitation-contraction coupling system.